Consider a single-level page table system, with the page table stored in the…
2020
Consider a single-level page table system, with the page table stored in the memory. If the hit rate to TLB is 80%, and it takes 15 nanoseconds to search the 𝑇𝐿𝐵, and 150 nanoseconds to access the main memory, then what is the effective memory access time, in nanoseconds?
Answer: B. 195 — Key insight: the TLB is always searched, and on a miss the page table must be read from memory before the data access. TLB search time (always): 15 ns If TLB…
- A.
185
- B.
195
- C.
205
- D.
175
Attempted by 203 students.
Show answer & explanation
Correct answer: B
Key insight: the TLB is always searched, and on a miss the page table must be read from memory before the data access.
TLB search time (always): 15 ns
If TLB hit (80%): one memory access for data = 150 ns, so total = 15 + 150 = 165 ns
If TLB miss (20%): must read page table (150 ns) and then data (150 ns), so total = 15 + 150 + 150 = 315 ns
Effective memory access time = 0.8 * 165 + 0.2 * 315 = 132 + 63 = 195 ns
Final answer: 195 ns
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